1. Field of the Invention
The invention relates to batteries and in particular to a method to manage charge and/or discharge of a rechargeable battery.
2. Related Background Art
Rechargeable batteries are being used in various applications. In rechargeable batteries, the chemical reactions which take place at the positive and negative electrodes in each electrochemical cell are reversible. As the electrochemical nature of battery systems differ significantly, different chargers are necessary to address the different types of battery charging needs.
Aircraft and other transportation applications typically make use of lead acid batteries in particular for starting an engine. It appears desirable to use other types of batteries in such applications instead of lead acid batteries in order to benefit from their advantages. In particular, Lithium Ion (Li-Ion) batteries are contemplated.
However, Li-Ion batteries require specific electronics to control the charge thereof especially to prevent overcharging. It has been observed that overcharging of Li-Ion batteries produces Lithium deposition on the negative electrode surface which accelerates aging of the battery. So, applications designed for working with lead acid batteries or other types of batteries are not adapted to charge safely Li-Ion batteries. In particular, the usual open circuit charge voltage of the battery charger or current-generating equipment used for lead acid batteries would overcharge a Li-Ion battery.
Typically, the maximum charging current that can be applied to a Li-Ion battery cell is limited by the maximum continuous current defined to avoid early aging of the battery. Thus, current pulses are limited by this maximum value even if a higher charging current during a very short time would not damage the electrochemical cell. When the battery is used in hybrid applications, it can be seek to store a large amount of energy during a short time but the charge controller would typically not allow a charge current higher than the maximum continuous current for the battery.
Charging of battery with current pulses has been used in low load applications such as mobile phones or desktop computers. U.S. Pat. No. 5,481,174 discloses such a method for rapidly charging a Lithium Ion cell according to a predetermined charging cycle.
U.S. Pat. No. 5,315,228 discloses a battery charge controller that calculates the state of charge dynamically for a nickel cadmium or nickel metal hydride electrochemical cell. The problem of maximum charging current is not hinted at in this document as it is specific to Lithium-Ion electrochemical cells.
U.S. Pat. No. 2006/0255766 discloses a method for managing a battery pack for hybrid or electric vehicles. Set conditions are defined. Charging can be carried out when current or voltage is within predetermined values otherwise a pre-charge relay is turned off and a warning signal is provided to the user. The predetermined values are defined and remain constant throughout the battery life.
U.S. Pat. No. 5,557,188 discloses a battery system with charge and discharge control. This document does not disclose how limit values are defined.
U.S. Pat. No. 7,164,256 discloses a method for determining if a requested electrical energy is available, for emergency breaking for instance. A battery model and a generator model are provided to determine the total power available in the vehicle electrical system. The battery model is updated continuously depending on aging. This document is however silent on how to define the maximum charging current allowed.
None of the above mentioned prior art document discloses a battery system allowing for a charge current higher than a maximum predetermined current defined for the battery. As mentioned before, when using Lithium-Ion battery, the charge current is strictly limited due to lithium plating constraints. This limitation of the charge current reduces the charge rate and prevents fast charging of the battery.
Therefore, there is a need for a battery charge controller that would allow a temporary charge current higher than the maximum continuous current defined for the battery. There is also a need for a method to manage the charge of a battery which would determine the instantaneous maximum charge current allowed without accelerating aging of the battery.
In the same way, discharge control is typically provided with any battery system to prevent too high discharge currents that would cause over heating of the electrochemical cells.
However, notably for hybrid applications, there are situations where high currents are required over a very short time but the discharge controller would typically not allow a discharge current higher than a maximum continuous current defined for the battery. This limitation of the discharge current reduces the available power that can be provided by the battery to the application.
Thus there is also a need to provide a solution for controlling the discharge current of a Lithium-Ion battery to allow a temporary discharge current higher than the maximum continuous current defined for the battery.